If the planned Geneva peace talks on Syria fail, it will mark the end of the road for attempts to find a negotiated solution to the conflict, and should open the door to the wholesale delivery of arms to the rebels, a Turkish minister has said.

Omer Celik, a close adviser to the Turkish prime minister on foreign policy, portrayed the Geneva meeting planned for mid-June as a last chance for the international community to salvage its credibility in the face of two years of slaughter in which more than 80,000 people have died.

Serious obstacles remain to the talks being held at all, however, including divisions in the international community over whether Iran should take part, and disarray among Syrian opposition groups.

On Wednesday evening the Syrian government confirmed for the first time that it would attend the peace talks. It has not decided on the makeup of its delegation. Russia has previously said it can guarantee a high-level presence from Damascus.

A senior UN official involved in preparing the talks told the Guardian that the biggest problem was getting the right people from both sides of the conflict to attend. "We need credible negotiating partners. This is the most important issue. This is not yet solved. The first condition concerns the quality of the talks," he said.

Russian officials have also warned that the lifting of the EU arms embargo, due to take effect on Saturday, undermined prospects for the talks. In a further sign of mounting friction in the runup to the Geneva meeting Sergei Lavrov, the foreign minister, also took aim yesterday at a US-backed resolution at the UN human rights council condemning the Syrian government. Lavrov described the resolution as odious and unwholesome.

Celik, who is Turkey's culture minister as well as being one of Prime Minister Recep Tayyip Erdogan's closest foreign policy advisers, said the Assad regime had been arming itself and receiving direct military support from Iran and the Lebanese Shia militia Hezbollah.

"If the Geneva meeting becomes just another means of gaining Assad some additional time, that will mean that all diplomatic initiatives will have been exhausted," he told the Guardian.

"If the Geneva meeting does fail, we believe the international community should take all necessary measures to support the protection of opposition forces," Celik said. Asked if that meant arming the opposition, he replied: "Of course."

Otherwise, he added: "Concepts of international community and international justice would be meaningless and the international community would just be standing by watching a dictator butcher his own people." He rejected objections that sending weapons to the rebel Free Syrian Army would add "fuel to the fire" in Syria and simply lead to escalation.

"We went through all the same arguments over Bosnia and in the time we went through those arguments 100,000 people died. Now we are doing it again, and 100,000 people have died in Syria," Celik said. Turkish officials draw close parallels between the Bosnian conflict in the 1990s and Syria. They see both as cases of the international community standing by in the face of the mass killing of Muslims, and argue that in both cases indifference has led to radicalisation.

The biggest hurdle to cross before the Geneva talks can convene is the international row over the participation of Iran, which is an active party to the conflict, sending Revolutionary Guards to train pro-government militias. It also backs Hezbollah, which has taken a key role on the government side of the war.

Russia is adamant that an Iranian delegation should be present. "The issue of Iran is key for us," Lavrov was quoted as saying by Russian news agencies. "Iran without question is one of the most important nations." Tehran has voiced readiness to attend and Turkey has backed the proposal."This can be overcome by negotiation because it is a must to have all the parties in that meeting if you want to it succeed," Celik said.

UN headquarters in New York is keen to see Tehran involved in the talks. "Iran should be there, but the resistance is very strong," said a senior UN official. "If we can't get Iran to the talks, we will need a parallel channel."

Saudi Arabia and France have led opposition to an Iranian presence. Riyadh has said it will boycott Geneva if Tehran takes part, while the French foreign minister, Laurent Fabius, said that to include Iran would be "extremely dangerous" as it would harm prospects of reaching a deal on its disputed nuclear programme.

"We fear that if they are part of the Syrian conference they will try to drag things on to such an extent that they will blackmail us saying that the Syrian crisis can only be resolved on condition that they have the nuclear bomb," Fabius told France Inter radio. Tehran denies having any aspirations to build a nuclear weapons.

A spokesman for the UK foreign office said discussions on participants at Geneva were still being negotiated but pointed to remarks by William Hague in parliament last week, in which the foreign secretary argued that Iran would have blocked any agreement at the first Geneva talks and that the UK's "starting assumption" is that the same countries which attended the first negotiations should come to the second. The US state department has said the issue is still being negotiated.

Deep divisions within the political opposition represent a stiff challenge to the Geneva conference organisers. A meeting of the National Coalition in Istanbul, which began on 23 May and was meant to last three days, is still deadlocked a week later on the first major item on its agenda, the admission of new members. That has to be resolved before new leaders are elected and a decision is taken on whether to go to the Swiss talks.

The military opposition is even more fragmented, with the UN special envoy, Lakhdar Brahimi, estimating that there as many as 1,400 different groups, battalions and brigades fighting government forces. Each currently has to negotiate for ammunition with an array of mostly Arab state and private backers.

However, a Syrian businessman involved in supplying rebel-held areas since the start of the uprising two years ago said the structure for distributing arms and ammunition is gradually being rationalised, with groups applying to a joint military commission for supplies for specific operations, and being judged by their effectiveness.

"It is getting to be a more rational streamlined setup," the businessman said. "You don't just turn up any more and say I have this many men, give me this many bullets."

• This article was amended on 4 June 2013. The original described Omer Celik as the foreign affairs chief in Turkey's ruling AK party. He was deputy chairman for foreign affairs until earlier this year. The photograph caption was also amended to identify Celik as the man on the left, not right.